This application relates to optical signal transmission and detection over dispersive optical links such as optical fibers.
An optical wave at an optical carrier frequency fc may be modulated at a subcarrier frequency fRF to produce two modulation sideband signals at frequencies of (fcxe2x88x92fRF) and (fc+fRF), respectively. The sideband signals may be used to carry information for transmission over an optical link or a network of optical links. The optical media in an optical link, e.g., optical fibers, may exhibit chromatic dispersion where spectral components at different frequencies in an optical signal can travel at different group velocities. Therefore, in optical systems where double-sideband signals are used, the two sideband signals at different frequencies of (fcxe2x88x92fRF) and (fc+fRF) in an optical signal may be delayed relative to each other.
One consequence of this delay between the sideband signals in the double-sideband optical signal is the fading or decay of the received subcarrier power, which varies as a function of the subcarrier frequency, the transmission distance in the fiber, and accumulated dispersion:             P              cl        ,        f              xc3x97                  cos        2            ⁡              (                                            ϕ              1                        +                          ϕ              2                                2                )              =            cos      2        ⁡          [              π        ⁢                  xe2x80x83                ⁢                              cLD            ⁡                          (                              f                                  f                  c                                            )                                2                    ]      
where xcfx861, and xcfx862 are respectively the phases of the modulation sidebands at (fcxe2x88x92fRF) and (fc+fRF) relative to the optical carrier at fc, c is the speed of light, L is the transmission distance, D is the dispersion parameter of the fiber in the link, and f is the subcarrier modulation frequency fRF. FIG. 1 illustrates this dispersion-induced power fading effect in a double-sideband optical signal.
This signal fading is undesirable in many applications because it can seriously deteriorate the detection of the optical signals. In some optical networks where the actual optical paths for transmitting signals may be reconfigurable, such power fading may dynamically change with the transmission distance. Therefore, it may be desirable to provide distance-independent power fading compensation in some microwave- and millimeter-wave-based optical systems that use double-sideband signals to transmit information.
The power fading compensation according to one embodiment includes optically splitting a double-sideband optical signal into a first optical signal in a first optical path and a second optical signal in a second optical path and then controlling dispersion in at least one of the first and the second optical paths to produce a difference of about xcfx80 between a sum of phases of the first and the second modulation sidebands relative to a phase of the optical carrier at the carrier frequency in the first optical signal, and a sum of phases of the first and the second modulation sidebands relative to a phase of the optical carrier in the second optical signal. A tunable optical dispersion element, such as a nonlinearly-chirped fiber Bragg grating, may be used to produce the desired dispersion.